1. Field of the Invention
The present invention is directed to forming composite parts and, more particularly, a vacuum assisted resin transfer molding (VARTM) apparatus and method for uniformly wetting out a fibrous pre-form by minimizing lateral travel of the uncured resin.
2. Description of Related Art
Known methods and apparatus have been used to impregnate dry fibrous materials by introducing a resin to the material under vacuum conditions, and then curing the resin to produce the composite part. One particular method employing a vacuum bag molding process, or vacuum assisted resin transfer molding (VARTM) process, includes placing a fiber lay-up in a rigid mold having a shape corresponding to the composite part being produced. Notably, such processes take advantage of a desired amount of compression provided by a vacuum bag used to enclose the resin impregnated fiber lay-up under vacuum pressure. To promote uniform and complete xe2x80x9cwet-outxe2x80x9d of the fiber lay-up, the vacuum operates to remove entrapped air in the lay-up as its formed into the composite structure. As a result, such systems generally avoid the creation of areas of the lay-up that are not infused with resin, which can compromise the structural integrity of the part. After wetting-out the fiber, the resin is then cured to complete the structure.
The lay-ups used in known systems for manufacturing solid and cored laminate composite structural members are typically made from glass or carbon fiber or other suitable fiber, or polyester cloth. To enhance structural characteristics of the member, such systems typically employ a number of plies of such fiber-reinforced material. Notably, known vacuum assisted resin transfer molding processes have been used to manufacture non-cored structures, as well as cored structures that include a core material disposed in the lay-up.
With more particular reference to making non-cored structures, a fibrous lay-up is initially placed in a self-contained mold having a desired shape. Then, typically, a resin distribution medium is placed on top of the lay-up. The medium separates the lay-up from a structure for maintaining vacuum pressure in the system, such as a flexible vacuum bag, and facilitates flow of uncured resin in the system by providing flow paths between the bag and the lay-up. The vacuum bag is fluid impervious and sealed to allow applied vacuum pressure to pull the resin through the fibrous lay-up, as noted previously. Also, a resin inlet is disposed, preferably, adjacent to the vacuum bag with the bag being sealed thereto to maintain vacuum pressure.
Similar apparatus is used to manufacture cored structures. However, the fibrous lay-up employed in manufacturing cored structures includes, typically, fiber-wrapped core structures made of, for example, balsa wood. As with the non-cored structures, the lay-up is then placed in a rigid mold and a vacuum bag is placed thereon and sealed thereto to maintain vacuum pressure. Further, a distribution medium may be employed, either between the core structures and the fibrous material wrapped thereon, or between the fiber wrapped cores and the vacuum bag, to promote uniform resin flow upon application of vacuum pressure. For both cored and non-cored structures, the resin in then cured and the composite part and the rigid mold are separated.
Current systems take 4-8 hours to disperse the resin through the fibrous lay-up. Notably, as the resin cures, it diminishes the effective area of the vacuum and in some instances seals areas in which resin has not fully infused the material. Where imperfect wetting occurs, the dry spots create inferior composite structures. As a result, some known systems attempt to create uniform flow of resin through the material in an effort to eliminate dry spots.
Generally, such systems position vacuum and resin ports in order to compensate for the effects of gravity, and to facilitate lateral movement of the resin. As a result, different portions of the fibrous structure may be infused. Typically, resin originating from one or more ports is caused to travel, for example, upwardly and laterally under pull from a vacuum source that is in a generally vertical direction, i.e., vacuum pressure is applied generally only in the Z direction. As the resin tends to move upwardly towards the vacuum port(s), the resin is dispersed in the X, Y and Z directions to infuse the lay-ups. To direct resin laterally from a narrow resin inlet channel prior to impregnating the lay-up, at least some known systems employ a disbursement medium. The medium creates channels that facilitate resin flow; however, much of the resin entering the mold still flows laterally (i.e., in the X and Y directions) within the lay-up. Typically, the resin moves as a wave, laterally infusing top portions of the lay-up before corresponding bottom portions, as the resin moves towards the vacuum port.
Overall, non-uniform resin flow results in different portions of the lay-up being infused with resin at different times. More particularly, resin travels the entire thickness of different lateral portions of the lay-up at different times, thus increasing the amount of process time required. Worse yet, this non-uniform, multi-directional resin flow increases the chances that unknown portions of the lay-up remain dry, thus creating weak spots and compromising the integrity of the finished part.
Therefore, the art is in need of an apparatus and method which eliminates or minimizes the existence of multi-direction flow of the resin to facilitate more uniform wet-out the fibrous pre-form. As a result, the apparatus should decrease the amount of time required to wet-out the fiber. Moreover, by minimizing or eliminating multi-directional resin flow, the apparatus and method should lessen the likelihood of dry spots in the resultant hardened composite part.
The preferred embodiment is directed to a vacuum assisted resin transfer molding (VARTM) process to produce fiber reinforced composite structures that facilitates more uniform and more time efficient resin infusion of the fibrous pre-form without requiring a distribution medium. In particular, the preferred embodiment uses a VARTM apparatus and method that maintains resin flow in the pre-form that is generally orthogonal, across the entire footprint of the pre-form, to a plane defined by a bottom surface of the corresponding rigid mold. As a result, the fibrous pre-form of the composite part is uniformly infused with resin, thus insuring the integrity of the completed part, yet correspondingly minimizing the time required to infuse the structure.
Accordingly, one aspect of the preferred embodiment includes an apparatus for molding a composite structure which includes an insert that is pervious to a flowable resin, and a mold having a cavity defining contiguous upper and lower portions. The upper portion is adapted to receive a fibrous pre-form, and the lower portion is adapted to receive the insert. Notably, the insert defines a volume to accommodate a pool of the flowable resin.
In accordance with another aspect of the invention, the insert includes a plurality of fluid transferable elements. The fluid transferable elements preferably include a plate having generally mutually spaced cylindrical openings and an expanded metal pre-form. The plate and the expanded metal pre-form are configured to facilitate resin flow in a direction generally orthogonal to a bottom surface of the mold to distribute the resin uniformly across the mold cavity.
According to another aspect of the preferred embodiment, the resin flows through the insert in a direction generally orthogonal to a planar interface between the lower portion and the upper portion of the cavity.
According to a further aspect of the preferred embodiment, an apparatus for use in molding a composite structure includes a rigid mold having a shape corresponding to a shape of the composite structure to be formed. The rigid mold is configured to receive a fibrous pre-form and an insert that defines a reservoir for containing a pool of resin. The insert is disposed generally adjacent to at least a portion of the rigid mold intermediate the rigid mold and the fibrous pre-form.
According to another aspect of the preferred embodiment, a method of vacuum assisted resin transfer molding includes the steps of providing a rigid mold having a shape corresponding to the part to be formed, and then positioning an insert in the mold to define a reservoir for containing a pool of resin, the insert including a plurality of mutually spaced openings. Next, the method includes positioning a fibrous pre-form on the insert, and then injecting resin through the mold to flood the reservoir.
According to another aspect of the preferred embodiment, the above-described method further includes sealing the mold with a vacuum bag prior to the injecting step and then applying vacuum pressure to the apparatus so as to uniformly pull the resin through the openings of the insert in a direction generally parallel to a corresponding longitudinal axis of each of the openings generally simultaneously across the entire planar interface between the insert and the fibrous pre-form. Then, curing the resin to complete the composite structure.
According to another aspect of the preferred embodiment, an apparatus for forming a composite structure using a VARTM process employs a rigid mold includes an insert disposed adjacent to a bottom surface of the rigid mold, wherein the insert facilitates resin flow in a direction generally orthogonal to the bottom surface.
These and other objects, features, and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.